Here's a little formula that I use:
Axle ratio x speed x trans ratio x 336.13 ÷ tire dia = RPM
What I came up with for my TH 350 tranny using a 3.08 rear gear and 26.5 in dia BFG Radial TA's . . .
MPH .. 3rd (1:1) .. 2nd (1.52:1) .. 1st (2.52:1)
30 ... 1,172 ... 1,781 ... 2,953
35 ... 1,400 ... 2,000 ... 3,500
40 ... 1,563 ... 2,375 ... 3,938
45 ... 1,750 ... 2,675 ... 4,400
50 ... 1,953 ... 2,969 ... 4,922
60 ... 2,350 ... 3,500 ... 5,907